Method of making a refractory material



asis nss Patented Jan. 7, 1958 METHOD F MAKENG A REFRACTORY MATERHALHerbert I. Miller, Los Alamos, N. Men, assignor to the United States ofAmerica as represented by the United States Atomic Energy Commission NoDrawing. Application June 23, 1949 Serial No. 100,963

3 Claims. (Cl. 1855) This invention relates to refractory materials and,more particularly, to such materials which have desired nuclearproperties for employment in the field of physics.

Since the discovery of nuclear fission, the importance of neutroninduced reactions in various materials has increased. it is well-knownat the present time, for example, that the nuclear characteristics ofcertain materials make them extremely desirable for use in connectionwith neutron emitting systems or the like and are useful generally inincreasing the efiiciency of such systems. Thus, in neutronic reactorssuch as described by Fermi and Szilard, U. S. Patent application S. N.568,904, filed December 19, 1944, now Patent No. 2,708,656, dated May17, 1955, it has been found that beryllium or compounds thereof, suchas, beryllium oxide, are extremely useful as neutron reflectingmaterials employed outwardly of the neutronic system to return to thesystem many of the neutrons which would otherwise escape therefrom. Thescattering cross-section of the beryllium nucleus for neutrons is knownto be very large and increasingly so as the energy of the neutrons isreduced. Furthermore, because its absorption cross-section for neutronsis extremely small, in fact negligible, beryllium being one of the lightelements is ideally adapted as a neutron moderator, that is, it willslow fast neutrons down to thermal energies very rapidly. See copendingU. S. patent application of Miller and Smith, S. N. 100,876, filed June23, 1949.

it is believed useful to explain in some detail the operation ofneutronic reactors or other neutron multiplying systems as a backgroundfor the present invention. It is well-known that certain elements, whensubjected to neutron bombardment, split or fission into two parts, i.e., into two elements having a lower atomic number and weight. Thisfission process is accompanied by the emission of various types ofradiation and nuclear particles. As a result of the fission of theoriginal nucleus, the new elements or fission products possess anextremely high velocity which may be expressed in terms of the kineticenergy thereof now known to be in the neighborhood of 160 millionelectron volts. Furthermore, the energy of the radiation emitted in theprocess is in the neighborhood of million electron volts. Theutilization of this energy forms the basis of the neutronic reactorsystem.

Among the particles emitted in the fission of such elements as uranium233, uranium 235 or plutonium 239 are at least two neutrons. It is thusseen to be possible that a self-sustaining neutron chain reaction can beestablished in which a fissile nucleus is bombarded by a neutron andsplits and, in turn, gives ofi further neutrons which may be employedfor bombarding further fissile nuclei and causing further fission. Asexplained in the above mentioned application of Fermi and Szilard, chainreactions can be maintained in systems in which normal polyisotopicuranium is employed as the reactive material. In such a system, abalance is obtained between the loss of neutrons by escape from thesystem or absorption in nuclei without producing fission and the gain inneutrons obtained by neutron induced fission of the uranium 235 nucleipresent as about one part in parts of uranium 238 in the normalpolyisotopic mixture. Such a balance is more readily obtained when theneutrons emitted in fission are slowed from their normal high energy tothermal energies at which the uranium 235 fission cross-section is verylarge. Such self-sustaining neutronic reactors employing normal uraniumgenerally comprise:

(l) A neutron slowing material known as a moderator, such as, graphite,in which the fissionable material is dispersed in a geometrical patterndesigned to reduce neutron losses.

(2) Heat removal means, for example, channels in heat exchangerelationship with the reactive mass and through which a suitable coolantis circulated in order to stabilize temperatures in the system.

(3) An outer casing sometimes called a tamper which serves to reflectneutrons back into the system and thereby reduce the quantity (i. e.,the critical mass) of fissionable mixture necessary to sustain thereaction.

(4) Means for charging the reactive elements into the zone in which thereaction takes place and for removal therefrom of the products of thereaction.

(5) A protective shield is sometimes provided around the reactor tominimize the escape of biologically harmful radiations. Such shields maycomprise, for example, bismuth or lead, which have been found effectivein stopping gamma. radiation, hydrogenous materials such as parafiin forabsorbing neutrons and/or massive outer concrete casing.

(6) A monitoring system to determine the reaction conditions at alltimes.

(7) Control devices generally comprising neutron absorbing materialsinsertable into the reactive mass to maintain an average state ofneutron production and adsorption balance at a predetermined level.

(8) The safety devices comprising a quantity of neutron absorbingmaterial which may be used to stop the reaction in case of emergency bybeing automatically inserted into neutronic absorbing relationship withthe re acting mass.

Large size reactors have been constructed in accord ance with the aboveand it has been found that components 2 and 4 in the above list raisedifficult problems in the construction. The temperatures created in thegeneration of the many kilowatts of power produced in such reactors areextremely high and intricate construction design features have beennecessary to prevent distortion of the system or any of the componentelements thereof. Furthermore, in order to prevent temperatures beyondthe limits of the structural materials employed, coolants have beenintroduced which create corrosion problems, contamination removaldiificulties and the like. Furthermore, the problem of charging such areactor is one of major consideration, because of the lattice structurenecessary when polyisotopic uranium is employed in the manner described.

Robert Christy in U. S. patent application S. N. 623,363, filed December12, 1944, has described a reactor in which the fissile material isemployed in the enriched state, for example, the percentage of uranium235 is increased to 30% instead of the 0.7% in normal uranium. Thisreactor may be made quite small, e. g., the fissile material thereinoccupying a zone about 12 inches in diameter. In the embodimentdescribed, the fissile material is em ployed in an aqueous solution, thehydrogen nuclei in the solution acting as the moderator to thermalizethe neu trons. Obviously, such a device must operate below the boilingpoint of the reactive solution thus limiting the power output.Furthermore, the desirable soluble fissile material compounds, e. g.,uranyl nitrate, form corrosive solutions which raise a number ofproblems in the construction and operational maintenance of the device.

It should also be noted that neutron multiplying systems may beconstructed which do not operate in a self-sustaining manner. Forexample, the prior art has disclosed a multiplying system in whichneutrons from an external source are allegedly multiplied in asubcritical quantity of fissile material. it is now well-known that aprimary neutron introduced into a subcritical system will be multipliedto a degree proportional to the ratio of the mass of fissile materialpresent to the critical mass value, and this relationship is employed inestablishing the critical mass value for various fissile materials.

It is an object of the present invention to provide a novel reactivecomposition for use in neutron multiplying systems and reactors such asdescribed above.

It is a further object of the present invention to provide reactivemixtures of fissile materials and moderating mate rials capable ofwithstanding extremely high temperatures and methods for making thesame.

It is a still further object of the present invention to providearrangements of fissile materials and moderating materials which havesuperior heat transfer characteristics.

Another object of the present invention is to provide a new article ofmanufacture and methods for making the same, the said article beingideally suited for employment in neutron multiplying systems by reasonof its elimination of corrosion problems heretofore encountered in suchsystems.

Still another object of the present invention is to provide a refractorymixture of a fissile material and a moderator material.

Other objects will become apparent to one skilled in the art from thefollowing description taken together with the illustrative exampleswhich are given by way of explanation and should not be deemed to belimitations on the scope hereof.

The above mentioned objects are attained according to the presentinvention by intimately mixing powdered oxides of a fissile material anda moderating material, pressing the said mixed oxides into subcriticalmasses of predetermined shape and density, and heating the said massesabove the sintering temperature thereof. The resultant product is acoherent refractory arrangement or brick of fissile nuclei substantiallyuniformly dispersed among moderating nuclei, the percentage of fissilenuclei present being predetermined to permit the employment of thepro-formed brick either by itself or in combination with other blocks(either of similar composition or size,

or not) or with neutron reflectors, absorbent shields or the like.Furthermore, by control of the composition according to fixed rules orotherwise, or according to predetermined Zones, circulation of coolantsthrough the refractory brick may be permitted or, alternatively, theheat conductivity of the brick provided for in a predetermined manner.

The fissile materials and other heavy metals of the second rare earthseries found useful in the present invention include uranium in itspolyisotopic form or the separated isotopes of uranium, such as, uranium233 or uranium 23 5, thorium and protactinium; the transuranic elements,neptunium, plutonium, amen'cium and curium or isotopes thereof. Thesematerials likewise may be admixed in predetermined quantities forparticular types of neutronic systems or employed in highly purifiedstates. The separated isotopes of these materials may be readilyobtained. For example, uranium 235 may be separated from uranium 238 inmass separating devices such as described by Robert R. Wilson in U. S.patent application S. N. 653,518, filed March 11, 1946, now Patent No.2,606,291, granted August 8, 1952. Another method of separating theuranium 235 isotope from the naturally occurring isotopic mixture is bygas diffusion methods employing uranium hexafluoride gas and diffusionbarriers.

In both methods, the separation is not completed in a single stage, butrather proceeds step-wise, or in cascade fashion, with the acceptedportion of each step being further separated and the rejected portionbeing re-cycled. It will thus be seen that the fissionable isotopeuranium 235 is observed to occur in greater abundance or concentrationwith each advancing step of the process, and the accepted portion ofeach step may be used in the practice of the present invention.

Uranium 233 may be formed by subjecting a quantity of thorium 232 toneutron bombardment, the resulting reaction being as follows:

235 min.

27.4 days 23 min. 92230 2.3 days as 94239 B The higher transuranicelements may be formed by wellknown transformation procedures.

It is preferred in the practice of the invention to employ thefissionable materials mentioned in the oxide form as this form has beenfound to possess the characteristics which make it a preferredrefractory material. The process for production of the oxides of thesematerials is now well-known and will not be discussed in detail herein.Furthermore, the manner of preparation of the oxides in predeterminedparticle sizes is also well-known at the present time.

The preferred moderating materials employed in the invention must alsopossess refractory characteristics. For this purpose, beryllium oxidehas been found most desirable. The presence of oxygen nuclei does notimpair the moderating action of the material. For example, at thermalenergies, the neutron cross-sections in units of 10 cm. of berylliumare, absorption 0.0085, scattering 6.1; for oxygen, absorption 0.0016and scattering 4.1.

It should be noted that when pure fissile isotopes, such as, uranium 233or uranium 235, or compositions including such isotopes enriched withrespect to other elements or isotopes are employed together withmoderating materials, small quantities of the said fissile material mayproduce a critical assembly and care must be taken to prevent thisresult through inadvertence or mistake. As a consequence, it isgenerally preferred to prepare the masses or bricks in relatively smallsizes when pure fissile materials-or enriched fissile materials areutilized.

Many of the possible variations in the articles, compositions andmethods which form the present invention will be apparent from thefollowing examples which also serve to indicate that the product formedby the practice of the present invention are largely defined by specificsteps in the process described.

Example 1 Minus 200 mesh particle size high fired beryllium oxide powderof normal commercial manufacture was intimately mixed in a standardmixer with uranium oxide having the composition U0 The mixture comprised95% beryllium oxide and 5% uranium oxide, the ma nium in this case beingthe normal isotopic mixture. After mixing for approximately 50 hours,water was added to the resultant mixed powder in a quantity sufficientto' form a stiff paste. Portions of the paste were then incorporatedinto a cylindrical die which permitted the application of a compressiveforce thereto and the elimination of part of the water from thecompacts, the compacting method employed being substantially thatdescribed in U. S. Patent 1,993,047 to Westman. The compacts wereextruded from the die and dried in air for 24 hours. Firing wasaccomplished in a high temperature furnace to which air was admitted andsaid firing was performed in accordance with a predetermined schedule.The temperature was first raised from room temperature to a temperatureof 1000 C. over a period of 30 minutes. At 1000 C., the temperature wasmaintained constant for 1 hour. Thereafter, the temperature was raisedas rapidly as possible to 1750' C. and maintained at that pointfor 3-hours. Upon the completion of this period, the current was turned offand the furnace permitted to cool to room temperature and the finishedbricks removed. After this treatment, the bricks were found to possess ahighly vitrified fine crystal structure and in compression tests werefound to have a crushing strength upward of 70,000 lbs. per square inch.Analysis indicated substantially uniform dispersion of uranium nucleiamong the beryllium nuclei.

Example 2 Beryllium oxide of standard commercial manufacture and ofminus 200 mesh particle size was intimately mixed with uranium oxide inthe form U 0 The proportions of the mixture were 98% beryllium oxide and2% uranium oxide by weight. The uranium oxide employed was enriched inthe isotope uranium 235 so that the said isotope was about 30% of theuranium present. The total weight of the batch was approximately grams.The subsequent steps followed those described in Example 1.

The resulting bricks were found to have a density of about 2.6 and werehighly vitrified and coherent, the crushing strength being upward of70,000 lbs. per square inch and tensile strength in the neighborhood of12,000 lbs. per square inch. Subsequently, bricks of this compositionwere heated to about 1300 C. for a period upward of 100 hours in orderto determine whether any density change occurred through prolongedheating such as would be encountered in a neutronic reactor. It wasfound that a reduction in density of about 2% resulted. Analysis of thesample thereafter showed substantially uniform dispersions of theuranium nuclei among the beryllium nuclei.

Example 3 Beryllium oxide and thorium oxide in powder form of standardcommercial manufacture were intimately mixed in a mixer for a period ofabout 50 hours. The mixture comprised 98% beryllium oxide and 2% thoriumoxide by weight. 25 gram portions of the said mixture of minus 200 meshparticle size was immersed in a solution of paraffin and naphtha. Theexcess liquid was poured off and the balance evaporated by warming theresultant sludge over boiling water. After evaporation, the mixture waspassed through a 150 mesh sieve and thereupon compacted in a cylindricalcompacting die under a pressure of 20,000 lbs. per square inch. Uponremoval from the die, the compacts were heated in a muffle furnace toabout 450 C. to drive off the paraffin. Thereafter, the temperature wasraised to about 1850" C. as rapidly as possible and held for 3 hours.The furnace was then permitted to cool to room temperature and thefinished brick removed therefrom. Analysis indicated that substantiallyuniform dispersion of the thorium nuclei and the beryllium nuclei wasobtained. The bricks in general were highly vitrified and coherent. Uponbreaking, they were found to possess a fine crystalline structure. Onesuch brick was irradiated in a neutronic reactor for about 100 hoursbeing subjected therein to a flux of about 10 neutrons per squarecentimeter per second. Subsequent checks indicated the presence of asubstantial number of uranium 233 nuclei and by reason of the presenceof these nuclei, which are alpha emissive, neutrons were detected in thevicinity of the brick resulting from the alpha-n reaction on theberyllium.

Example 4 One of the bricks produced in accordance with Example 1 wascrushed in a standard crusher. 20% by weight minus 200 mesh powder andby weight of particles larger than 40 mesh were incorporated in a dieafter being treated with parafiin as in Example 3. A pressure of 5,000lbs. per square inch is applied in the forming process and the resultantporous compact carefully extruded from the die. This compact was firedby first raising the temperature to about 400 C. to drive off theparafiin and then subsequently raising the temperature as rapidly aspossible to about 1800 C. where it was held for about 3 hours. Thetemperature was then reduced to room temperature at a uniform slow rateand the resultant brick removed from the furnace. While the crushingstrength of this sample was low, it possessed the desirable property ofpermitting substant-ial quantities of gas or air to pass therethrough atreasonably low pressures.

Example 5 A cylindrical tube of beryllium oxide was fabricated inaccordance with the procedures outlined in U. S. patent application Ser.No. 641,618, filed January 16, 1946 by George D. Cremer. The tube was 1inch long, /2 inch inside diameter and 1% inches outside diameter. Acompact was formed within the beryllium oxide tube by incorporating thetube in a close-fitting die, tamping the paste of uranium oxide andberyllium oxide mixture within the inner bore of said tube and thenapplying a mechanical compressive pressure through suitable plungers tosaid paste. The pressure employed was 10,000 lbs. per square inch. Theescape of excess liquid was prevented in order to increase the radialpressures and assist in obtaining a good bond between the inner compactand its supporting beryllium oxide tube. Thereafter, the compositecompact was heated in air to about 1950 C. and the temperaturemaintained constant for 3 hours. The furnace was then permitted to coolto room temperature and the composite brick removed therefrom. Uponexamination, a good bond between the inner mixture and the outerberyllium oxide tube was found to exist, both of said components beinghighly vitrified and of fine crystal structure. Obviously, instead ofemploying a prefabricated tube of beryllium oxide, a preliminary coldcompacting step would have been feasible and, in fact, would result in abetter bond between the components.

From the above examples, it is apparent that a highly useful compositionof matter has been produced in accordance with the principles herein setforth. Obviously, many variations in the materials employed and in thecompositions by weight or atomic percent are possible depending upon thedesired use of the products now made available. The temperature setforth above in the illustrative examples were predetermined for highlyvitrified products capable of withstanding substantial pressures. Lowertemperatures will result in reduced densities and reduced crushingstrengths. It should be noted that when firing is accomplished attemperatures in excess of 1300" C., while the above mentionedcharacteristics are changed, none of the various bricks thus preparedshowed material spalling, cracking or other deficiencies under repeatedheat cycles in which the maximum temperature was 1000 C. Furthermore,temperatures as high as 2000 C. have been employed in the a firingprocess without introducing undesirable characteristics in the finishedproduct.

In fact, when raw compacts made as described above were heated inclosed, unlined graphite molds at such temperatures (around 2000 C.),the formation of a substantial amount of beryllium carbide and uraniumcarbide was found to have taken place at the surface of the vitrifiedbrick, thus resulting in a product containing beryllium oxide, berylliumcarbide, uranium oxide and uranium carbide. Obviously, uranium carbideand beryllium carbide can be substituted for the oxides of theseelements in making the raw compacts described in the examples, and thecarbide form of either element may be mixed with the oxide of the other,or oxides and carbides of both combined, to render like results whenfired.

Repeated tests in which the bricks formed as above described, weresubjected to various types of radiation such as one would encounter in aneutronic reactor showed no material crystal deterioration and thetransition from substantially nonfissionable isotopes to fissionableisotopes, as described in connection with Example 3, was accomplishedwithout any detectable change in the structure or the othercharacteristics of the bricks.

Obviously, when such transformations take place, for example, thetransformation from uranium 238 through neptunium to plutonium 239, itmust be recognized that an alpha emitting isotope has been formed andthat neutrons will be emitted by the constituents of the brick.

Thus, beyond producing a neutron source which would be very useful formany purposes, the methods disclosed herein provide steps in whichrelatively harmless materials from the radioactive standpoint may befabricated into predetermined bodies and then made radioactive in theircompleted form eliminating hazardous machining steps and the like. It isthus also seen that many widely different variations and modificationswill suggest themselves to one skilled in the art and that the same maybe made without departing from the spirit of the scope of the inventionas defined in the appended claims.

What is claimed is:

1. The process which comprises intimately mixing powders consistingessentially of a refractory oxide of a metal selected from the groupconsisting of thorium, uranium and plutonium and of beryllium oxide,adding paraffin to the mixed powders, compacting the mixture underpressure of 5000 p. s. i., heating the compact to drive oil the paraflinand then heating in the presence of air the compact to at least thesintering temperature to form a unitary structurally strong compact.

2. The process of claim 1 in which the metal oxide is uranium oxide.

3. The process of claim 1 in which the metal oxide is thorium oxide.

References Cited in the file of this patent UNITED STATES PATENTS873,568 Magoon Dec. 10, 1907 1,430,724 DAdrian Oct. 3, 1922 2,200,258Boyer May 14, 1940 2,205,308 Pirani June 18, 1940 2,448,479 Wilhelm etal. Aug. 31, 1948 FOREIGN PATENTS 173,237 Great Britain Mar. 21, 1923114,150 Australia May 2, 1940 114,151 Australia May 3, 1940 861,390France Oct. 28, 1940 233,011 Switzerland Oct. 2, 1944 OTHER REFERENCESWebsters New International Dictionary, p. 433, G. & C. Merriam Co.,Springfield, Mass. (1929).

Roberts et al.: Uranium and Atomic Power, J. Applied Physics, vol. 10,pp. 612-614, September 1939.

Chemical Abstracts, p. 7734 (1940), abst. of Zeldovich and Kharitonarticle in J. Exptl. Theoret. Phys. (U. S. S. R.) 10, 29-36 (1940).

Hopkins: Chapters in the Chemistry of the Less Familiar Elements, vol.II, page 7, Stipes Pub. Co. (1939).

Hackhs Chemical Dictionary, 3rd ad, page 775, Blakiston (1944).

Goodman The Science & Eng. of Nuclear Power, vol. I, pp. 302 and 303,Addison-Wesley Press (1947), Cambridge, Mass, pp. 325 and 326.

Powder Metallurgy by Paul Schwarzkopf, New York, McMillan Co. (1947 pp.44, 207, 208.

Kelly et al., Phy. Rev. 73, 1135-9 (1948).

1. THE PROCESS WHICH COMPRISES INTIMATELY MIXING POWDERS CONSISTINGESSENTIALLY OF A REFRACTORY OXIDE OF A METAL SELECTED FROM THE GROUPCONSISTING OF THORIUM, URANIUM AND PLUTONIUM AND OF BERYLLIUM OXIDE,ADDING PARAFFIN TO THE MIXED POWDERS, COMPACTING THE MIXTURE UNDERPRESSURE OF 5000 P.S.I., HEATING THE COMPACT TO DRIVE OFF THE PARAFFINAND THEN HEATING IN THE PRESENCE OF AIR THE COMPACT TO AT LEAST THESINTERING TEMPERATURE TO FORM A UNITARY STRUCTURALLY STRON COMPACT.